1. Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to an optical device that is thermally isolated from the surrounding environment and which has enhanced mechanical strength.
2. Related Art
Wavelength division multiplexing (WDM) is widely used to communicate modulated data at different carrier wavelengths on a common optical waveguide. WDM can overcome optical-fiber congestion, which is a potential problem in optical modules that include parallel optical transceivers with one channel per optical fiber. In particular, by significantly reducing the number of optical fibers per optical module, WDM multiplexing can simplify optical modules, thereby reducing their cost and size.
In dense WDM (DWDM), a narrow spacing between adjacent wavelengths is used. This is typically achieved by modulating data directly onto a highly stable optical carrier, and then combining multiple carriers in an optical fiber. DWDM allows a large number of channels to be accommodated within a given wavelength band, and thus offers high performance.
In DWDM, a variety of optical devices are used as: modulators, multiplexers (such as add filters), de-multiplexers (such as drop filters), filters and switches. In order to compensate for fabrication variation, temperature variation and/or laser wavelength drift, the operating wavelengths of the optical devices (such as the resonant wavelength of a ring resonator) are typically phase-tuned to target wavelengths corresponding to the channels in a DWDM link. Depending on the system requirements, a tuning range of at least 180° may be needed.
Thermal tuning is a popular tuning technique because it provides the ability to produce large phase shifts. Existing thermal tuning techniques include direct heating (which is implemented by doping in an optical waveguide) and indirect heating (in which a heater is proximate to the optical waveguide). Typically, the direct-heating technique is more energy-efficient than indirect heating, but it can prevent the optical waveguide from performing additional functions (because of the constraint on the doping density), and it can introduce additional optical losses due to free-carrier absorption (which can degrade the quality factor of an optical resonator).
In principle, optical devices can be made on silicon substrates, because silicon provides many benefits for optical communication. For example, the high index-of-refraction contrast between silicon and silicon dioxide can be used to create sub-micron waveguides to confine light with spatial densities that are up to 100× larger than in a single-mode optical fiber. Furthermore, by using a silicon-on-insulator (SOI) technology, a silicon waveguide can be surrounded by silicon dioxide on all four sides, which facilitates low-loss, on-chip waveguides and active devices (such as detectors and modulators). Silicon-based optical devices can be used to implement a wide variety of optical components for use in WDM communication. These silicon-based optical devices offer numerous advantages, including: miniaturization, low-energy modulation, the ability to integrate with other devices in silicon, and/or the ability to leverage the large, existing silicon manufacturing infrastructure.
Nonetheless, there are problems associated with silicon-based optical devices. A notable problem is heat dissipation in the silicon, as well as in the top metal and the dielectric stack. While the high thermal conductivity of silicon helps remove the heat dissipated by electrical circuits, it can make it more difficult to thermally tune a silicon-based optical device. In particular, because the operating wavelength of a silicon-based optical device (such as the resonant wavelength of an optical resonator) strongly depends on temperature, the operating wavelength is typically tuned using either direct or indirect heating to change the operating temperature of the silicon-based optical device. However, the high thermal conductivity of silicon results in excessive thermal coupling to the surrounding environment. Consequently, thermal tuning of silicon-based optical devices often consumes a disproportionately large amount of energy (typically, 50-100 mW for a phase shift of 180°). This high power consumption can offset the advantages provided by silicon, and makes it more difficult to use silicon-based optical devices to implement optical communication (such as WDM) in computing systems (especially in systems that have multiple instances of the optical devices).
One existing approach to address this problem is to increase the thermal isolation of a silicon-based optical device (thereby reducing the thermal-tuning power) by removing at least a portion of the silicon substrate proximate to the optical device, thus creating a free-standing portion of the optical device. For example, the silicon substrate may be micro-machined to create a backside etch pit. However, the free-standing portion of the optical device is mechanically unsupported, which can result in problems during subsequent processing. In particular, following completion of wafer-scale fabrication, silicon substrates are typically subjected to manufacturing operations, such as: wafer-scale testing, wafer-scale bumping, die singulation by mechanical sawing or laser dicing, and packaging (for example, flip-chip integration by thermocompression bonding and wirebonding). These manufacturing operations often include temperature cycles that place mechanical stress on the mechanically unsupported portions of the optical device, such as the back-end-of-line (BEOL) interlayer dielectric (ILD) stack. These manufacturing challenges are expected to be increasingly important as SOI technologies are scaled to include fragile, ultra-low dielectric constant ILD stackups for improved performance.
Hence, what is needed is an optical device that can be thermally tuned without the above-described problems.